1. Field of the Invention
The present invention relates to a transformer based power conditioner to provide high purity electrical power, preferably having capacitor based circuitry on the transformer secondary to provide noise filtering, and preferably having semiconductors on the transformer primary functioning as suppressors for transient electrical surges.
2. Description of the Related Art
High purity electrical power generally means that the power is substantially free from voltage spikes and sags with no significant neutral-to-ground voltage. A number of electronic devices require such high purity power. Among them are medical imaging systems such as X-rays, computer tomography, magnetic resonance imaging, and radiation treatment systems. All of these devices require a large amount of current but only for a short duration, that is, when the X-ray or magnetic generator is operational. The power during this exposure must be clean for good image quality. Additionally, the stand-by power between exposures must also be clean for the reliable operation of the computerized control and imaging processing subsystems, which operate between exposures. An example of a power conditioner to provide high purity electrical power is shown in U.S. Pat. No. 5,012,382.
For these types of systems, the voltage drop during the exposure period should be minimal, typically less than about 8%. This voltage drop is a result of the impedance of all upstream wiring, connections and transformers in the circuit. The reason for this limitation on voltage drop is that the exposure duration in the medical imaging systems is often calculated based on the magnitude of the line voltage present immediately before exposure. Significant changes in this voltage during exposure can result in unpredictable dosages. It is also important that operation of the generator does not produce voltage sags or spikes on the power lines which interfere with the reliable operation of other system components.
Other systems requiring high purity power include automated test equipment and telecommunications equipment. Automated test equipment (ATE) is used in a number of applications, one of which is the manufacture of semiconductors and printed circuit boards. For example, during the testing of semiconductors (like computer microprocessors) the ATE provides an array of inputs to the microprocessor and detects the response and response speed to those inputs. Impure power can cause the ATE to interpret the responses incorrectly, and as a result, eliminate good parts or retain bad parts. Printed circuit board (PCB) ATE provides an array of inputs to PCBs and determines if the traces have continuity to the desired areas of the PCB. Impure power can cause the ATE to give erroneous indications about PCB continuity. Manufacturers of such equipment continue to reduce the semiconductor and PCB test voltages to reduce the power requirements, allowing the ATE to be smaller and more cost effective for the end user. However, with lower operating test voltages, the susceptibility of the ATE to impure power becomes greater and the need for cleaner power increases.
Telecommunications equipment also requires clean power for similar reasons. Clean power allows the communications equipment to transmit with higher quality. This results in better sound and data quality and fewer dropped connections. Another advantage is that clean power ensures that telecommunications equipment will continue to operate without interruption for longer periods.
Surge suppression devices or L-C filters, or both, on the building wiring near the load provide another way to achieve the desired power quality. These devices shunt impulses above certain voltage or frequency levels from one wire to another. They typically are comprised of metal oxide varistors (MOVs), silicon avalanche diodes (SADs), gas discharge tubes, capacitors and inductors, and often incorporate resistors. An example of a transient voltage surge suppressor using MOVs and silicon surge suppression diodes is discussed in U.S. Pat. No. 4,802,055. Another example of MOVs used for electrical transient suppression is described in U.S. Pat. No. 5,038,245. Still another example of a way to suppress transients is discussed in U.S. Pat. No. 4,156,838, which does not employ magnetic coupling.
There are several limitations with many of these types of devices and filters. They shunt away voltage spikes or dips (xe2x80x9cnormal mode noisexe2x80x9d), but in doing so increase the current in the neutral conductor, creating neutral-ground potentials (xe2x80x9ccommon mode noisexe2x80x9d) which can be even more damaging or disruptive than normal mode noise. Their effects are limited since they can only protect to a certain voltage or frequency level. For example, MOV""s and avalanche diodes xe2x80x9cwear outxe2x80x9d with time and lose their effectiveness.
For this protection scheme to be as effective as possible, the inductance in series with the shunt elements (surge suppressors and capacitors in L-C filters) must be minimized. The wire length connecting the suppressors to the conductors makes a measurable difference in their effectiveness. Often, these devices are connected to the power lines by wires which are five to 50 feet in length due to physical placement constraints in the field or limited knowledge on the part of the installers, or both. The length of the wires between the power lines and the suppressors also limits the effectiveness of scaling the product for optimal performance. Surge suppressors are also frequently sized inappropriately to simplify installation at the expense of performance.
Also limiting the performance of shunt elements is the lack of ability to improve performance by cascading the filtering elements. Many of these filters employ single stage filtering that limits the effectiveness of the elements within the suppressors.
In an attempt to minimize lead length, maximize performance, and make site performance consistent, series power line filters have been used in these applications. Many of these devices have a series element (typically an inductor) that adds impedance to the line and increases cost significantly, but improves high frequency filtering performance greatly. The added series impedance has negative effects on power quality, particularly voltage regulation, during high current demand, as described earlier. It also limits the cost effectiveness of scaling these types of products.
Another alternative solution is to use a conventional shielded isolation transformer. The shield in the transformer increases the isolation of the output from the conducted ground (common mode) noise. A neutral-ground bond converts common mode noise to normal mode noise and allows a more effective use of filters and surge suppressors on the transformer as described above.
The extra impedance of the series power line filters (with inductors) or shielded isolation transformer methods results in lower power quality when the filters or transformers interact with computer loads. Modern computers have xe2x80x9cswitch modexe2x80x9d power supplies which draw their current in short bursts where the change in current with respect to time (di/dT) is fast, being the equivalent to 120 Hz or greater, instead of the usual 60 Hz for most conventional loads. Even at low load factors, these rapidly varying loads cause conventional transformers to produce outputs with a flat-topped voltage waveform and voltage spikes. Other switching transients within the system only add to the problem.
Often shielded isolation transformers and suppression/filtering devices are combined in the field in an attempt to provide the quality power desired. The limitations mentioned above also apply to this combination.
In contrast to the use of passive shunt element based solutions, other attempts to provide high purity power for these systems have utilized conventional voltage regulation schemes. Those attempts have generally not been successful. Electronic-controlled tap-switching voltage regulators are undesirable because the step voltage changes produced by tap changes during exposure often cannot be tolerated. Motorized variac solutions are typically not fast enough to compensate for most common power quality problems. Saturable-core ferroresonant transformers are also unacceptable as they have very high impedance and slow reaction time. They interact with the pulsed load by creating large voltage transients. None of these approaches can effectively produce the power quality required by the systems described earlier.
Thus the challenge in the prior art is to provide high quality power as previously defined to a load having relatively high peak pulsed power and moderate average power requirements, with a relatively small, economical power conditioner.
The present invention solves the problems stated by means of an improved power conditioner circuit which includes a special type of power line filter system having a low series impedance at both 60 Hz and at high frequencies when compared to conventional filters. This low impedance filter, coupled with a low impedance isolation transformer, provides superior power quality for loads with high peak pulsed power requirements. The power line filter system can be cascaded to increase performance in a cost-effective manner. The filter system may include suppressor elements in some embodiments.
This power line filter system incorporates several features not found in conventional filter systems. One feature of the invention is that the filter for each phase is connected to the secondary of the transformer before the phase-to-phase bond. The neutral conductors of each phase""s filter are physically separated from one another to minimize coupling of high frequency components from input to output of the filter. A second feature is the specific way of internally wiring two or more separate shunt capacitors within the individual filter module. The shunt capacitors and their connections are physically separated from one another to minimize coupling of high frequency components from the input to the output of the filter module. Another feature is that the location of the filter inside the power conditioner enclosure delivers consistent, high performance of the system from site to site because the wiring is as short as possible, typically less than two inches and preferably approaching zero length. Also because of the compact nature of the power circuitry in the power conditioner enclosure, the wiring can be maintained in a production-controlled environment to ensure production consistency.
Another feature of the power line filter system of the invention when suppressor elements are included is the manner in which the suppression elements are connected to each primary winding. The suppressors are connected across the entire tapped winding, regardless of which tap is connected to the power line input. This connection scheme advantageously uses the magnetic coupling of the extended windings that are electrically connected to the input power line windings. Connecting the suppressor in this manner clamps transients more effectively than placing the suppressor directly across the power line input.
Transformers are often made with multiple primary sections for connection in series or in parallel to accommodate the different standard input voltages worldwide. A further feature of the invention is the use of an individual suppressor across each of the multiple primary winding sections in the transformer. The magnetic coupling between the primary sections causes the suppressors to share the current accompanying a high voltage transient, providing more reliable performance and a longer life for the suppressors.
One aspect of the present invention involves a transformer-based filtered power conditioning system, the system including a system ground or neutral. The system has a transformer having primary and secondary windings with the primary windings having input connections and the secondary windings having output connections. Input power lines are connected to the primary winding input connections and are adapted to be connected to a source of electrical power. In one embodiment, surge suppression elements are connected across the ends of each primary winding. Preferably, the surge suppression elements are connected to the start and finish points for each coil in the primary, regardless of tap selection for the input power.
In one embodiment, at least one filter circuit is provided, comprising at least one input shunt leg and at least one output shunt leg, each shunt leg having at least one resistor and at least one capacitor between two connections, thereby forming R-C circuits. Advantageously, a very low impedance element is connected between the connections of the input and output shunt legs, wherein a first connection of the input shunt leg is connected to one polarity of the secondary windings, a second connection of the input shunt leg is connected to the opposite polarity of the secondary windings; a first connection of the output shunt leg is one polarity of the output of the power conditioning system and is adapted to be connected to a load or to a first connection of an input shunt leg of a second filter circuit, and a second connection of the output shunt leg is the opposite polarity of the output of the power conditioning system and is adapted to be connected to a load and optionally connected to system neutral and ground, or to a second connection of an input shunt leg of a second filter circuit.
In one embodiment, the suppression elements are selected from the group consisting of metal oxide varistors (MOVs), silicon avalanche diodes (SADs), gas discharge tubes, and capacitor and inductor combinations, the combinations selectively including resistors. In one embodiment, the capacitors in each shunt leg are substantially equal in capacitance, and in another embodiment, the capacitors in each shunt leg are unequal in capacitance.
In one embodiment, the filter circuit further has an inductor.
Advantageously, the low impedance connecting the ends of the shunt legs comprises an electrically conductive element to which the shunt legs are mounted with the mountings being spaced apart on the electrically conductive element. The system is also preferably scalable by adding additional filter circuits to the secondary winding outputs.
In one embodiment, the filter circuits have an anti-parallel diode arrangement connected across each resistor in each shunt leg. In one embodiment, the filter circuit is connected between the secondary windings and the output neutral, and the connection to system ground is a phase-to-phase neutral bond
The power conditioning system is a three-phase system in one embodiment, but may also be a single-phase system in another embodiment, and is adaptable to any number of phases.
Other additional features of the invention which provide further improvements are described below.